Resonant CO2 sensing with mitigation of cross-sensitivities

Abstract

Various exemplary embodiments relate to a device to measure carbon dioxide (CO 2 ) levels, including a first oscillator group comprising a first sensor to measure air pressure, where the first sensor comprises a first sealed membrane, and where the first sealed membrane overlays a sealed first cavity; a second oscillator group including a second sensor to measure the resonance frequency of a second unsealed oscillating membrane, and where the second unsealed membrane overlays a second cavity in contact with the air outside of the second sensor; and a mixer accepting as input a first frequency measurement output from the first oscillator group and a second frequency measurement output from the second oscillator group, outputting the difference of the first frequency measurement and the second frequency measurement, and computing a carbon dioxide measurement based on the difference.

What is claimed is: 
     
       1. A device to output a difference of frequency measurements, comprising:
 a first oscillator group comprising a first sensor to measure air pressure, wherein the first sensor comprises a first sealed membrane, wherein the first sealed membrane overlays a sealed first cavity; 
 a second oscillator group comprising a second sensor to measure the resonance frequency of a second unsealed oscillating membrane, wherein the second unsealed membrane overlays a second cavity in contact with the air outside of the second sensor; and 
 a mixer accepting as input a first frequency measurement output from the first oscillator group and a second frequency measurement output from the second oscillator group, and outputting a difference of the first frequency measurement and the second frequency measurement. 
 
     
     
       2. The device of  claim 1 , wherein the first sealed membrane and the second unsealed oscillating membrane measure 200×200 μm 2 . 
     
     
       3. The device of  claim 1 , wherein the first sealed membrane is hermetically sealed and deflects under a pressure gradient. 
     
     
       4. The device of  claim 3 , wherein deflecting under a pressure gradient comprises deflecting under a pressure difference between inside the membrane and outside the membrane. 
     
     
       5. The device of  claim 1 , wherein the second oscillator group further comprises a first top electrode and a second bottom electrode. 
     
     
       6. A device to output a difference of frequency measurements, comprising:
 a first sensor to measure air pressure, wherein the first sensor comprises a sealed membrane, wherein the sealed membrane overlays a sealed first cavity; 
 a second sensor to measure the resonance frequency of an unsealed oscillating membrane, wherein the second sensor comprises a heater, wherein the unsealed membrane overlays a second cavity in contact with the air outside of the second sensor, and wherein the heater is in contact with the interior surface of the second cavity; and 
 a mixer accepting as input a first frequency measurement output from the first sensor and a second frequency measurement output from the second sensor, and outputting a difference of the first frequency measurement and the second frequency measurement. 
 
     
     
       7. The device of  claim 6 , wherein the sealed membrane and the unsealed oscillating membrane measure 200×200 μm 2 . 
     
     
       8. The device of  claim 6 , wherein the sealed membrane is hermetically sealed and deflects under a pressure gradient. 
     
     
       9. The device of  claim 6 , wherein the second sensor further comprises a first top electrode and a second bottom electrode.